Introduction: The human capacity for explosive force (EF) production is considered functionally more important than maximal voluntary force (MVF) during explosive movements, such as sprinting, jumping or when restabilising the musculo-skeletal system following a loss of balance and thus in preventing injury [1]. Fatigue is thought to increase injury risk [2], however, the effect of fatigue on EF and the mechanisms responsible (neural or contractile) have received little attention. This study assessed the neural and contractile contributions to fatigue during EF production. Method: Eleven healthy untrained males (24 +- 4 yrs) completed 10 sets of voluntary maximal explosive isometric contractions of the knee extensors (5 x 3-s duration, 2-s rest). Sets were separated by 5-s, during which twitch and octet (8 pulses at 300 Hz; evokes the muscles maximal rate of force development [RFD]) contractions were electrically evoked via supramaximal stimulation of the femoral nerve. EF was measured over the initial 50 (Octet) and 150 ms (volitional) of contraction. Peak octet force (PF) and MVF were also assessed. Voluntary EF was reported in absolute and relative (%MVF) terms. Surface EMG RMS amplitude was recorded over the superficial agonists (VL, VM and RF) during the initial 150 ms of contraction and at MVF (500 ms epoch). EMG was normalised to Mmax Area during the same set of contractions before being averaged across the agonists. To assess fatigue paired t-tests were used to i) compare measurements during the initial vs. final sets and ii) contrast the fatigue induced reduction in different parameters. Results: Voluntary EF after 50-150 ms of contraction declined by 47-52% with a 42% reduction in MVF (All, P < 0.001). Relative EF decreased by 12- 18% (P . 0.038). The decline in octet force at 50 ms (23%, P<0.001) and PF (28%, P<0.001) were less than the equivalent changes in voluntary force (All, P . 0.027). Agonist EMG during the initial 150 ms of contraction and at MVF decreased by 15-28% (All, P . 0.010). Conclusion: EF exhibited a marked decline with fatigue, which was more pronounced than the decrease in MVF, as shown by the reduction in relative EF. This finding may help explain the greater incidence of injuries associated with fatigue (i.e., at the end of a football match). Contractile fatigue was demonstrated by declines in Octet EF after 50 ms and at PF, but these changes were smaller than volitional decrements. The decline in agonist EMG and the greater decrease in voluntary than Octet EF demonstrated a substantial central neural component to the reduction in volitional EF.
© Copyright 2012 17th Annual Congress of the European College of Sport Science (ECSS), Bruges, 4. -7. July 2012. Veröffentlicht von Vrije Universiteit Brussel. Alle Rechte vorbehalten.